1. Field of the Invention
The present invention relates to a method for pretreating a catalyst comprising at least one Group VIII noble metal and a support material prior to its use in a catalytic reforming process. The pretreatment passivates the activity of the catalyst during the initial stages of reforming.
2. Description of the Prior Art
Catalytic reforming is a process for converting hydrocarbon feedstocks containing mainly saturated hydrocarbons to more valuable aromatic compounds. Typically, naphtha, a mixture of paraffins, naphthenes and aromatics, is reformed to produce gasolines with high octane number. The reforming process is generally carried out by passing the naphtha over a suitable catalyst in the presence of hydrogen at elevated temperatures and pressures. The catalyst employed is generally platinum carried on a support material, for example alumina, silica, silica-alumina or crystalline aluminosilicates (zeolites). The catalyst may also include a second metal component, for example rhenium, tin or another Group VIII noble metal such as iridium or rhodium.
The reforming of naphtha involves several different reactions including isomerization, dehydrogenation of naphthenes to aromatics, dehydrogenation of paraffins to olefins, dehydrocyclization of paraffins and olefins to aromatics, and hydrocracking of paraffins to gaseous hydrocarbons such as methane and other lower alkanes. Ideally the reforming process minimizes the hydrocracking of paraffins and maximizes the reactions leading to the formation of more valuable products, particularly dehydrocyclization and dehydrogenation to aromatics.
During the initial stages of the reforming process the platinum-containing catalyst exhibits a high degree of hydrocracking activity leading to excessive formation of undesirable light hydrocarbons. As well as giving a lower yield of desired aromatic compounds, the hydrocracking reaction has the disadvantage that it is highly exothermic and can lead to temperature runaway in the reactor.
To lower the initial hydrocracking activity of the catalyst, it is known to passivate the catalyst by pretreating it with a sulfur-containing fluid such as hydrogen sulfide gas or an organic sulfide. This treatment has the disadvantage that it involves the use of a toxic, pungent, corrosive substance. Also, when the support material is a zeolite, especially a zeolite containing 1-dimensional channel-shaped pores such as a type L zeolite, the sulfur leads to excessive deactivation of the catalyst.
Other passivation techniques have been suggested in the past. U.S. 3,592,780 discloses a pretreatment method in which a platinum catalyst is contacted with naphtha containing at least 200 ppm sulfur in the presence of hydrogen at elevated temperatures and pressures for a number of hours. In U.S. 3,438,888 the pretreatment method consists of contacting a platinum-rhenium catalyst with a highly aromatic hydrocarbon stock at reforming conditions, in the presence of hydrogen, at elevated temperatures and pressures. This pretreatment is carried out for at least 0.5 hour and preferably a number of hours. To our knowledge neither of the pretreatment processes disclosed in these two patents has been used commercially.